Studies of murine antibody gene organization have shown that the heavy chain variable region (VH) is composed of several gene families. The usage of these gene families appears to be random in adult mice by Northern analysis of B cell hybridomas and transformed cell lines. The technique for B cell colony growth in filter paper disks has been established in our laboratory as described in a previous report. We have used this colony hybridization technique to examine VH gene expression in polyclonally-activated cells from normal CBA/Ca mice and from a spontaneously derived mutant line of CBA/Ca, CBA/N, which is unresponsive to polysaccharide antigens. The results of the analysis of CBA/CA indicate a pattern of VH gene usage which is random and thus proportional to the size of the gene family. Out data are consistent with published data from another laboratory of similar studies of CBA/J mice. When we examined CBA/N mice, however, we found two striking differences from CBA/Ca. First, the overall percent of IgM expressing colonies (approximately 90% of LPS-induced colonies) detected by probes for 9 VH gene families was 90% in CBA/Ca but only 70% in CBA/N, indicating that a significant proportion of their VH genes are different from CBA/Ca. Second, CBA/N had a reduced expression of the largest family, J558, compared to CBA/Ca. Third, whereas J558 as expected, was the predominant family in 9 of 10 CBA/Ca mice, it was the predominant family in only 2 of 12 CBA/N mice, indicating the non-stochastic VH gene expression in the defective strain. Studies are in progress to examine the expression of VH gene families in CBA/N mice during ontogeny. Preliminary results indicate that in the neonatal CBA/N, like the adult, VH gene expression is not normal. The extent to which these defects relate to the inability of the CBA/N mouse to mount an antibody response to polysaccharide antigens remains to be determined.